Platelet rich plasmas are tested using the Capture-P kit (Immucor). Platelet monolayers in microtiter wells are incubated with anti-PL- A1, washed, and exposed to anti-immunoglobulin G-coated red blood cells. The coated red blood cells adhere to PL-A1 positive platelets but not to PL-A1 negative platelets. Centrifugal sedimentation of unattached red blood cells produces a central button, indicating that platelets are PL-A1 negative. Of 520 donors, 15 (2.88%) tested PL-A1 negative, which correlates well with the reported frequency of PL-A2 in caucasians in the literature. All 15 PL-A1 negative donors were confirmed by allele-specific restriction assays. Confirmatory genotyping was not done on those donors testing PL-A1 positive. The rate of PL-A1 negative donors correlates well with the published rate for caucasians. The assay can be used to screen large numbers of samples. Confirmatory testing can be reserved for sample testing PL-A1 negative by solid phase. Ongoing work involves identifying the rate of "false positives."